Such a light-detecting device, which normally comprises, as described for example in patent EP-0 633 457, a matrix of photosensors which are arranged in rows and in columns and which are each able to take one of two states, an active state when they are illuminated and an inactive state when they are not illuminated, is more particularly, although not exclusively, designed for detecting a very-short-duration laser pulse which is emitted in an ambient light environment of variable illumination, for example at a predetermined frequency.
By way of illustration, the laser pulse to be detected may be emitted by a laser source which is aimed at a target, for example a tank, in order to highlight it, so that it can be located by a weapons system, such as a missile, which is equipped with said light-detecting device and which is designed to destroy this target. To this end, said light-detecting device is for example associated with the homing head of the missile, has a fixed position on said missile and observes the space to the front of said missile. The information relating to the location of the target, which information is generated by this light-detecting device, is supplied to the homing head which uses it to guide the missile onto the target.
A major drawback concerning the use of such a light-detecting device for guiding a missile lies in the difficulty and in the considerable duration needed to extract the zone which is illuminated by the laser pulse from said matrix of photosensors, and to locate this zone on the matrix, so as to obtain information concerning the position of the target, which information is necessary for guiding the missile. Because of the high speed of the missile, it is essential for any change in position to be detected rapidly in order to be able to supply correction orders in time to the components driving said missile, so that it does not miss the target.
Now, to detect the position of a light spot, that is to say the position of the aforementioned illuminated zone, on the matrix of photosensors, it is known for the state (active or inactive) of each photosensor to be analyzed individually. Such an analysis, if it is implemented photosensor by photosensor, is very lengthy, and in any case too lengthy for the applications envisioned above, for which the information must be available very quickly.
Moreover, it is also known to use a logic OR gate of the “global OR” type over the entire matrix, which makes it possible to indicate whether at least one photosensor is in the active state on said matrix by simultaneous analysis of all the photosensors. However, such a “global OR” does not make it possible to identify the photosensor or photosensors which are activated, and therefore to locate them. Such a “global OR” does therefore not allow the rapid extraction of an illuminated zone from a matrix of photosensors.